Chemical Equilibrium and Le Chatelier's Principle

• Understand dynamic equilibrium and what it means at the molecular level
• Write equilibrium expressions Kc and Kp for any reaction
• Relate Kp and Kc through Δn
• Use the reaction quotient Q to predict shift direction
• Apply Le Chatelier's principle to changes in concentration, pressure, and temperature
• Understand why a catalyst does not change K

Level 1 — Foundations

Dynamic equilibrium: At equilibrium, the forward and reverse reactions continue at equal rates. Concentrations remain constant — not zero — for both reactants and products.

Equilibrium Constant Kc: For the general reaction: aA + bB ⇌ cC + dD

Kc = [C]^c [D]^d / ([A]^a [B]^b)

Rules for writing Kc:

• Only include species in solution or gas phase (not pure solids or pure liquids)
• Concentrations in mol/L raised to stoichiometric power
• Kc is dimensionless in strict thermodynamic terms, but NCERT/JEE use units implicitly

Large Kc (>> 1): products favoured at equilibrium Small Kc (<< 1): reactants favoured at equilibrium

Level 2 — JEE depth

Relationship between Kp and Kc: Kp = Kc (RT)^Δn

where:

• Δn = (moles of gaseous products) − (moles of gaseous reactants)
• R = 0.0821 L atm mol⁻¹ K⁻¹
• T in Kelvin

If Δn = 0: Kp = Kc If Δn > 0: Kp > Kc If Δn < 0: Kp < Kc

Reaction Quotient Q: Same expression as Kc, but using current (non-equilibrium) concentrations.

Le Chatelier's Principle: "If a system at equilibrium is subjected to a change, it will shift in the direction that opposes the change."

Responses to disturbances:

1. Adding reactant: Q < K → forward shift
2. Removing product: Q < K → forward shift
3. Increasing pressure (decreasing V): shifts toward side with fewer moles of gas (reduces Δn to relieve pressure)
4. Decreasing temperature: shifts toward exothermic direction (system produces heat to compensate)
5. Increasing temperature: shifts toward endothermic direction; K itself changes
6. Adding catalyst: no shift, equilibrium reached faster, K unchanged
7. Adding inert gas at constant V: no shift (partial pressures unchanged)
8. Adding inert gas at constant P: volume increases → effective dilution → shift toward more gas moles side

Temperature and K — van't Hoff equation: ln(K₂/K₁) = −(ΔH°/R)(1/T₂ − 1/T₁)

For exothermic reaction (ΔH < 0): K decreases as T increases For endothermic reaction (ΔH > 0): K increases as T increases

Haber process application: N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ/mol (exothermic)

• High pressure: favours NH₃ (4 mol → 2 mol gas)
• Low temperature: favours NH₃ (K increases) but rate is too slow
• Compromise: 400–500°C, 150–200 atm, Fe catalyst

JEE trap: When volume is halved (pressure doubled), Q ≠ K because concentrations of all species double. Calculate new Q and compare to K to find the shift direction.

JEE trap: For heterogeneous equilibria (e.g., CaCO₃ ⇌ CaO + CO₂), solid concentrations are not included in K. Kp = P(CO₂) only.

N₂ + 3H₂ ⇌ 2NH₃, Kc = 0.5 M⁻² at 400°C. At equilibrium [N₂] = 2 M, [H₂] = 3 M. Find [NH₃].

Kc = [NH₃]² / ([N₂][H₂]³)

0.5 = [NH₃]² / (2 × 3³)
0.5 = [NH₃]² / (2 × 27)
0.5 = [NH₃]² / 54

[NH₃]² = 0.5 × 54 = 27

[NH₃] = √27 = 3√3 ≈ 5.196 M ≈ 5.2 M

Answer: [NH₃] ≈ 5.2 M

PCl₅ ⇌ PCl₃ + Cl₂, Kp = 1.5 atm at 250°C. Find Kc.

T = 250 + 273 = 523 K
Δn = (1 + 1) − 1 = 1
R = 0.0821 L atm mol⁻¹ K⁻¹

Kp = Kc(RT)^Δn
1.5 = Kc × (0.0821 × 523)^1
1.5 = Kc × 42.94

Kc = 1.5 / 42.94 = 0.0349 M

Answer: Kc ≈ 0.035 mol/L

• Q1: Write Kc for: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
• Q2: For the above reaction, Kc = 280 and Kp = ? at 1000 K. Find Kp.
• Q3: If Kc = 4.0 and Q = 6.5, which direction does the reaction shift?
• Q4: For N₂O₄ ⇌ 2NO₂ (endothermic), predict the effect of: (a) increasing temperature, (b) increasing pressure, (c) adding a catalyst.
• Stretch: Q5: At 500 K, Kc = 0.061 for H₂ + I₂ ⇌ 2HI. If 1 mol H₂ and 1 mol I₂ are placed in a 10 L flask, calculate equilibrium concentrations of all species using ICE table.

NCERT Chapter 7 link: Chapter 7 (Class 11) — "Equilibrium" covers dynamic equilibrium, the law of mass action, Kc and Kp, relationship between them, Q vs K, and Le Chatelier's principle with industrial applications. Pay special attention to Examples 7.1–7.8 and the Haber process discussion.

Exam connections: JEE Mains tests Kc/Kp conversions, Q vs K predictions, and Le Chatelier disturbance MCQs. JEE Advanced tests ICE table problems, multi-step equilibria, and van't Hoff equation calculations linking K at two temperatures.

Study strategy: Build a Le Chatelier response table (stress → direction of shift → new equilibrium) and fill it in for 5 different reactions. Then practise ICE table problems — they appear in almost every JEE paper.

• Use the platform-native live simulation or PhET-style tool for this topic.
• Mirror / body / home activity: dissolve salt in water to saturation, then add more salt (precipitate forms) — observe Le Chatelier in action; photograph and describe.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.